U.S. patent application number 12/979059 was filed with the patent office on 2011-06-09 for nitride semiconductor light-emitting device and method for fabrication thereof.
This patent application is currently assigned to SHARP KABUSHIKI KAISHA. Invention is credited to Masahiro Araki, Takeshi KAMIKAWA, Yoshika Kaneko, Eiji Yamada.
Application Number | 20110136276 12/979059 |
Document ID | / |
Family ID | 34106865 |
Filed Date | 2011-06-09 |
United States Patent
Application |
20110136276 |
Kind Code |
A1 |
KAMIKAWA; Takeshi ; et
al. |
June 9, 2011 |
NITRIDE SEMICONDUCTOR LIGHT-EMITTING DEVICE AND METHOD FOR
FABRICATION THEREOF
Abstract
A nitride semiconductor laser device uses a substrate with low
defect density, contains reduced strains inside a nitride
semiconductor film, and thus offers a satisfactorily long useful
life. On a GaN substrate (10) with a defect density as low as
10.sup.6 cm.sup.-2 or less, a stripe-shaped depressed portion (16)
is formed by etching. On this substrate (10), a nitride
semiconductor film (11) is grown, and a laser stripe (12) is formed
off the area right above the depressed portion (16). With this
structure, the laser stripe (12) is free from strains, and the
semiconductor laser device offers a long useful life. Moreover, the
nitride semiconductor film (11) develops reduced cracks, resulting
in a greatly increased yield rate.
Inventors: |
KAMIKAWA; Takeshi;
(Mihara-Shi, JP) ; Yamada; Eiji; (Mihara-Shi,
JP) ; Araki; Masahiro; (Mihara-Shi, JP) ;
Kaneko; Yoshika; (Funabashi-Shi, JP) |
Assignee: |
SHARP KABUSHIKI KAISHA
Osaka-shi
JP
|
Family ID: |
34106865 |
Appl. No.: |
12/979059 |
Filed: |
December 27, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10902481 |
Jul 30, 2004 |
7903708 |
|
|
12979059 |
|
|
|
|
Current U.S.
Class: |
438/31 ;
257/E21.215; 438/42 |
Current CPC
Class: |
B82Y 20/00 20130101;
H01L 33/007 20130101; H01S 2304/04 20130101; H01S 5/2201 20130101;
H01S 5/0213 20130101; H01S 5/2231 20130101; H01S 2304/12 20130101;
H01S 5/2214 20130101; H01S 5/34333 20130101 |
Class at
Publication: |
438/31 ; 438/42;
257/E21.215 |
International
Class: |
H01L 21/306 20060101
H01L021/306 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 31, 2003 |
JP |
2003-204262 |
Jun 22, 2004 |
JP |
2004-183163 |
Claims
1. A method for fabricating a nitride semiconductor laser device,
comprising: a first step of forming a substrate whose top surface
is a nitride semiconductor layer and that has a carved region and a
non-carved region; a second step of forming, on the top surface of
the substrate, a nitride semiconductor film having a stripe-shaped
laser light waveguide structure, wherein the top surface of the
substrate has a low defect region with a defect density of 10.sup.6
cm.sup.-2 or less, in the first step, the carved region is formed
in the top surface of the substrate in the low defect region of the
substrate, and the non-carved region is formed in the top surface
of the substrate. in the second step, the nitride semiconductor
film is formed above both the carved region and the non-carved
region, in the second step, the laser light waveguide structure in
the nitride semiconductor film is formed right above the non-carved
region, and in the second step, the laser light waveguide structure
in the nitride semiconductor film is formed in a region located 5
.mu.m or more away from the nearest edge of the carved region in
the top surface of the substrate.
2. A method for fabricating a nitride semiconductor light-emitting
device, comprising: a first step of forming a substrate whose top
surface is a nitride semiconductor layer and that has a carved
region and a non-carved region; a second step of forming, on the
top surface of the substrate, a nitride semiconductor film having a
light-emitting region, wherein the top surface of the substrate has
a low defect region with a defect density of 10.sup.6 cm.sup.-2 or
less, in the first step, at least one carved region is formed in
the top surface of the substrate in the low defect region of the
substrate, and the non-carved region is formed in the top surface
of the substrate, in the second step, the nitride semiconductor
film is formed above both the carved region and the non-carved
region, in the second step, the light-emitting region in the
nitride semiconductor film is formed right above the non-carved
region, and in the second step, the light-emitting region in the
nitride semiconductor film is formed in a region located 5 .mu.m or
more away from the nearest edge of the carved region in the top
surface of the substrate.
3. A method for fabricating a nitride semiconductor laser device,
comprising: a first step of forming a substrate whose top surface
is a nitride semiconductor layer and that has a carved region and a
high-flatness region; a second step of forming, on the top surface
of the substrate, a nitride semiconductor film having a
stripe-shaped laser light waveguide structure and a depressed
portion, wherein the top surface of the substrate has a low defect
region with a defect density of 10.sup.6 cm.sup.-2 or less, in the
first step, the carved region is formed by etching in the top
surface of the substrate in the low defect region of the substrate,
in the second step, the nitride semiconductor film is formed on the
substrate, in the second step, the depressed portion is formed only
in a part of the nitride semiconductor film right above the carved
region, in the second step, the laser light waveguide structure in
the nitride semiconductor film is formed right above the
high-flatness region, and in the second step, the laser light
waveguide structure in the nitride semiconductor film is formed in
a region located 5 .mu.m or more away from the nearest edge of the
carved region in the top surface of the substrate.
4. A method for fabricating a nitride semiconductor light-emitting
device, comprising: a first step of forming a substrate whose top
surface is a nitride semiconductor layer and that has a carved
region and a high-flatness region; a second step of forming, on the
top surface of the substrate, a nitride semiconductor film having a
light-emitting region and a depressed portion, wherein the top
surface of the substrate has a low defect region with a defect
density of 10.sup.6 cm.sup.-2 or less, in the first step, the
carved region is formed by etching in the top surface of the
substrate in the low defect region of the substrate, in the second
step, the nitride semiconductor film is formed on the substrate, in
the second step, the depressed portion is formed only in a part of
the nitride semiconductor film right above the carved region, in
the second step, the light-emitting region in the nitride
semiconductor film is formed right above the high-flatness region,
and in the second step, the light-emitting region in the nitride
semiconductor film is formed in a region located 5 .mu.m or more
away from the nearest edge of the carved region in the top surface
of the substrate.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 10/902,481, filed Jul. 30, 2004, which claims priority of
Japanese Patent Application Nos. 2003-204262, filed Jul. 31, 2003
and 2004-183163, filed Jun. 22, 2004, the contents of which are
incorporated herein by reference in their entireties.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a nitride semiconductor
light-emitting device, and to a method for fabricating one. More
particularly, the present invention relates to a nitride
semiconductor light-emitting device that uses a nitride
semiconductor as a substrate thereof.
[0004] 2. Description of Related Art
[0005] There have been fabricated prototypes of semiconductor laser
devices that oscillate in a region ranging from ultraviolet to
visible light by the use of a nitride semiconductor material as
exemplified by GaN, AlN, InN, and composite crystals thereof. For
such purposes, GaN substrates are typically used, and therefore GaN
substrates have been intensively researched by a host of
research-and-development institutions. At the moment, however, no
semiconductor laser devices offer satisfactorily long useful lives,
and accordingly what is most expected in them is longer useful
lives. It is known that the useful life of a semiconductor laser
device strongly depends on the density of defects (such as
vacancies, interstitial atoms, and dislocations, all disturbing the
regularity of a crystal) that are present in a GaN substrate from
the beginning. The problem here is that substrates with low defect
density, however effective they may be believed to be in achieving
longer useful lives, are extremely difficult to obtain, and
therefore researches have been eagerly done to achieve as much
reduction in defect density as possible.
[0006] For example, Applied Physics Letter, Vol. 73 No. 6 (1998),
pp. 832-834, reports fabricating a GaN substrate by the following
procedure. First, on a sapphire substrate, a 2.0 .mu.m thick primer
GaN layer is grown by MOCVD (metalorganic chemical vapor
deposition). Then, on top of this, a 0.1 .mu.m thick SiO.sub.2 mask
pattern having regular stripe-shaped openings is formed. Then,
further on top, a 20 .mu.m thick GaN layer is formed again by
MOCVD. Now, a wafer is obtained. This technology is called ELOG
(epitaxially lateral overgrown), which exploits lateral growth to
reduce defects.
[0007] Further on top, a 200 .mu.m thick GaN layer is formed by
HVPE (hydride vapor phase epitaxy), and then the sapphire substrate
serving as a primer layer is removed. In this way, a 150 .mu.m
thick GaN substrate is produced. Next, the surface of the obtained
GaN substrate is ground to be flat. The thus obtained substrate is
known to have a defect density as low as 10.sup.6 cm.sup.-2 or
less.
[0008] It has been found, however, that, even with a semiconductor
laser device fabricated by growing, by a growing process such as
MOCVD, a nitride semiconductor film on a low-defect substrate, such
as the one obtained by the above-described procedure, it is still
impossible to obtain a useful life that is satisfactorily long for
practical use. Through an intensive study in search of the reason
for that, the inventors of the present invention have found out
that strains and cracks present inside a nitride semiconductor film
greatly affect the deterioration and yield rate of a semiconductor
laser device. Even when a GaN substrate that is homoepitaxial with
a nitride semiconductor film is used, the grown nitride
semiconductor film includes layers of InGaN, AlGaN, and the like
whose lattice constants and thermal expansion coefficients differ
from those of GaN. The presence of these layers different from GaN
causes an InGaN active layer and other layers to receive
compressive stress. It has been found that the resulting strains
present inside the film accelerate the deterioration of the
semiconductor laser device.
[0009] Another problem with a nitride semiconductor film is many
cracks that develop therein, resulting in a low yield rate. The
development of such cracks is also greatly affected by strains
present inside the film.
[0010] More specifically, when a laser structure formed of a
nitride semiconductor thin film is epitaxially grown on a nitride
semiconductor substrate, many cracks (for example, several or more
within a width of 1 mm) develop, resulting in an extremely low
yield rate of devices with the desired characteristics. If a
fabricated device contains cracks, the device may be flatly unable
to produce laser oscillation at all, or, even if it can, its useful
life is extremely short, making the device practically unusable.
The development of such cracks is remarkable in a device structure
including a layer containing Al, and, since a nitride semiconductor
laser device typically includes such a layer, it is very important
to eradicate cracks.
SUMMARY OF THE INVENTION
[0011] In view of the conventionally encountered problems mentioned
above, it is an object of the present invention to provide a
nitride semiconductor light-emitting device, such as a nitride
semiconductor laser device, that uses a substrate with low defect
density, that contains reduced strains inside a nitride
semiconductor film, and that thus offers a satisfactorily long
useful life. It is another object of the present invention to
provide a method for fabricating such a nitride semiconductor
light-emitting device with a high yield rate.
[0012] To achieve the above objects, in one aspect of the present
invention, in a nitride semiconductor laser device including a
substrate of which at least a surface is a nitride semiconductor
and a nitride semiconductor film laid on top of the surface of the
substrate and having a stripe-shaped laser light waveguide
structure, the surface of the substrate has a low-defect region
with a defect density of 10.sup.6 cm.sup.-2 or less and a depressed
portion, and the laser light waveguide structure of the nitride
semiconductor film is located above the low-defect region and off
the depressed portion of the surface of the substrate.
[0013] In this nitride semiconductor laser device, a substrate
having a depressed portion formed in a surface thereof is used, and
a laser light waveguide structure formed in a nitride semiconductor
film is located off the area right above the depressed portion of
the substrate. When the nitride semiconductor film is grown, in the
depressed portion of the substrate, growth progresses from
different directions and a defect develops where growth from
different directions meets, but, elsewhere, growth progresses
regularly, suppressing meeting of growth accompanied by defects. In
the area above the low-defect region and off the depressed portion,
there are less defects resulting from defects of the substrate, and
new defects are also less likely to develop, making presence of
strains unlikely. By locating the laser light waveguide structure
of the nitride semiconductor film in this strain-free area, it is
possible to give the device a long useful life. Moreover, even if
cracks develop, their development is limited within an area away
from the laser light waveguide structure. This helps achieve an
increased yield rate.
[0014] Advisably, the depressed portion of the surface of the
substrate is stripe-shaped. This makes it possible to suppress,
over a wide area, meeting of growth accompanied by defects. Thus,
it is possible to locate the stripe-shaped laser light waveguide
structure surely in an strain-free area.
[0015] Advisably, the depressed portion of the surface of the
substrate is given a horizontal cross-sectional area of 30
.mu.m.sup.2 or more. This helps further reduce strains.
[0016] Alternatively, the depressed portion of the surface of the
substrate is given a horizontal cross-sectional area of from 5
.mu.m.sup.2 to 30 .mu.m.sup.2, both ends inclusive, and the nitride
semiconductor film is given a thickness of from 2 .mu.m to 10
.mu.m, both ends inclusive. The smaller the depressed portion of
the substrate is, and the thicker the nitride semiconductor film
is, the less effectively the depressed portion reduces strains in
the nitride semiconductor film. With the cross-sectional area of
the depressed portion set within the range defined above, and in
addition with the thickness of nitride semiconductor film set
within the range defined above, it is possible to satisfactorily
reduce strains.
[0017] Alternatively, the surface of the substrate has, as the
depressed portion, a plurality of depressed portions arranged at
intervals of from 50 .mu.m to 2 mm, both ends inclusive. How
effectively the depressed portion reduces strains in the nitride
semiconductor film depends on the distance from the depressed
portion. With the interval at which the plurality of depressed
portions are arranged set within the range defined above, it is
possible to locate the laser light waveguide structure surely in an
strain-free area.
[0018] Preferably, the center of the laser light waveguide
structure of the nitride semiconductor film is located 5 .mu.m or
more away from an edge of the depressed portion of the surface of
the substrate. In an area within the nitride semiconductor film
close to the area thereof located above the depressed portion,
strains may develop under the influence of meeting of growth
accompanied by defects. With the laser light waveguide structure
located so far away as defined above, it is possible to locate the
laser light waveguide structure surely in an strain-free area.
[0019] To achieve the above objects, in another aspect of the
present invention, a method for fabricating a nitride semiconductor
laser device including a substrate of which at least a surface is a
nitride semiconductor and a nitride semiconductor film laid on top
of the surface of the substrate and having a stripe-shaped laser
light waveguide structure includes the steps of: forming a
depressed portion on the substrate, which includes on the surface
thereof a low-defect region with a defect density of 10.sup.6
cm.sup.-2 or less and; and locating the laser light waveguide
structure of the nitride semiconductor film above the low-defect
region and off the depressed portion of the surface of the
substrate.
[0020] Here, the substrate having the low-defect region and the
depressed portion may be produced by forming, on a first nitride
semiconductor having a low-defect region, a layer of a second
nitride semiconductor and then removing at least part of the second
nitride semiconductor.
[0021] To achieve the above objects, in still another aspect of the
present invention, in a nitride semiconductor laser device
including a substrate of which at least a surface is a nitride
semiconductor and a nitride semiconductor film laid on top of the
surface of the substrate and having a stripe-shaped laser light
waveguide structure, the surface of the substrate has a depressed
portion, and the laser light waveguide structure of the nitride
semiconductor film is located above a region located off the
depressed portion of the surface of the substrate.
[0022] Alternatively, in a nitride semiconductor light-emitting
device including a substrate of which at least a surface is a
nitride semiconductor and a nitride semiconductor film laid on top
of the surface of the substrate and having a light-emitting region,
the surface of the substrate has a depressed portion, and the
light-emitting region of the nitride semiconductor film is located
above a region located off the depressed portion of the surface of
the substrate.
[0023] Here, the depressed portion of the surface of the substrate
may be stripe-shaped and formed in a mesh-like pattern.
[0024] To achieve the above objects, in a further aspect of the
present invention, a method for fabricating a nitride semiconductor
light-emitting device including a substrate of which at least a
surface is a nitride semiconductor and a nitride semiconductor film
laid on top of the surface of the substrate and having a
light-emitting region includes the steps of: forming, on the
surface of the substrate, a depressed portion; forming, on the
surface of the substrate having the depressed portion formed
therein, the nitride semiconductor film; and locating the
light-emitting region above a region located off the depressed
portion of the surface of the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIGS. 1A and 1B are a sectional view and a top view,
respectively, schematically showing the structure of a nitride
semiconductor laser device embodying the invention;
[0026] FIGS. 2A and 2B are a top view and a sectional view,
respectively, schematically showing the structure of the GaN
substrate used in the nitride semiconductor laser device;
[0027] FIG. 3 is a top view schematically showing the wafer having
a nitride semiconductor grown on a conventional GaN substrate;
[0028] FIG. 4 is a sectional view schematically showing the layer
structure of the nitride semiconductor film of the nitride
semiconductor laser device;
[0029] FIG. 5 is a top view schematically showing the wafer having
a nitride semiconductor grown on the GaN substrate used in the
nitride semiconductor laser device;
[0030] FIG. 6 is a diagram showing the correlation between the
depression cross-sectional area, the nitride semiconductor film
thickness, and the useful life test yield rate;
[0031] FIGS. 7A and 7B are a sectional view and a top view,
respectively, schematically showing the nitride semiconductor laser
device with the nitride semiconductor film thereof made
thicker;
[0032] FIGS. 8A and 8B are a top view and a sectional view,
respectively, schematically showing another structure of the GaN
substrate used in the nitride semiconductor laser device;
[0033] FIGS. 9A to 9C are sectional views each schematically
showing a different pattern of the carved regions formed in the GaN
substrate;
[0034] FIG. 10 is a top view showing the surface morphology of the
GaN substrate;
[0035] FIG. 11 is a diagram showing height variations on the
surface of the nitride semiconductor;
[0036] FIG. 12 is a diagram showing height variations on the
surface of the nitride semiconductor; and
[0037] FIGS. 13A and 13B are a top view and a sectional view,
respectively, schematically showing the structure of LEDs formed on
the carved substrate.
DETAILED DESCRIPTION OF THE INVENTION
[0038] Hereinafter, embodiments of the present invention will be
described with reference to the accompanying drawings. In the
following description, a negative index in a formula indicating a
plane or direction of a crystal will be represented by a negative
sign "-" followed by the absolute value of the index instead of the
absolute value accompanied by an overscore placed thereabove as
required by convention in crystallography, simply because the
latter notation cannot be adopted in the present specification.
Example of how a Substrate is Produced
[0039] Part of the fabrication process of a low-defect GaN
substrate (with a defect density of 10.sup.6 cm.sup.-2 or less)
used in this embodiment can be achieved by the procedure described
earlier in connection with the conventional example. Specifically,
first, on a sapphire substrate, a 2.5 .mu.m thick primer GaN layer
is grown by MOCVD. Then, on top of this, a SiO.sub.2 mask pattern
having regular stripe-shaped openings (with a period of 20 .mu.m)
is formed. Then, further on top, a 15 .mu.m thick GaN layer is
formed again by MOCVD. Now, a wafer is obtained.
[0040] The film does not grow on SiO.sub.2, and thus starts to grow
inside the openings. As soon as the film becomes thicker than the
SiO.sub.2, the film then starts to grow horizontally away from the
openings. At the center of every SiO.sub.2 segment, different
portions of the film growing from opposite sides meet, producing,
where they meet, a defect-concentrated region with high defect
density. Since the SiO.sub.2 is formed in the shape of lines,
defect-concentrated regions are also formed in the shape of lines.
As described earlier, this technology is called ELOG, which
exploits lateral growth to reduce defects.
[0041] Further on top, a 600 .mu.m thick GaN layer is formed by
HVPE (hydride vapor phase epitaxy), and then the sapphire substrate
serving as a primer layer is removed. Next, the surface of the
obtained GaN substrate is ground to be flat. In this way, a 400
.mu.m thick GaN substrate is produced.
[0042] The substrate thus obtained has the GaN layer grown
extremely thick, and thus includes, over almost the entire area
thereof, low-defect regions with a defect density of 10.sup.5
cm.sup.-2 or less. However, depending on the growth conditions,
regions with a defect density higher than 10.sup.5 cm.sup.-2 may be
formed in the shape of stripes on the surface of the obtained
substrate in such a way as to correspond to the defect-concentrated
regions mentioned above. It should be noted that, also during
growth by HVPE, by forming a SiO.sub.2 mask above the
defect-concentrated regions, it is possible to more effectively
reduce defects on the surface of the substrate.
[0043] Here, the substrate is produced by ELOG. It should be
understood, however, that how the substrate is produced does not
affect in any way the nature and effects of the present invention.
Specifically, the only requirement is to use a nitride
semiconductor substrate having a low-defect region on the surface
thereof.
[0044] The dislocation density can be evaluated by one of the
following and other methods. One way is by subjecting the substrate
to etching by dipping it in a mixed acid liquid, namely a mixture
of sulfuric acid and phosphoric acid, heated to 250.degree. C. and
then counting the EPD (etch pit density) within a 50 .mu.m.times.50
.mu.m region. Another way is by counting the dislocation density in
an image obtained by using a transmission electron microscope.
[0045] The measurement of the EPD can be made possible by the use
of gas-phase etching such as RIE. Alternatively, suspension of
growth in a MOCVD furnace followed by exposure to a high
temperature (about 1,000.degree. C.) also makes the measurement of
the EPD possible. The measurement itself can be achieved by the use
of an AFM (atomic force microscope), CL (cathode luminescence),
microscopic PL (photo luminescence), or the like.
[0046] It should be noted that a substrate with low defect density
denotes not only a substrate having low-defect regions distributed
all over the area thereof but also a substrate including low-defect
regions in only a portion of the surface thereof. The low-defect
regions may be distributed in any manner, but the laser stripes of
semiconductor lasers need to be so formed as to include those
low-defect regions.
Forming a Semiconductor Laser Device
[0047] By the procedure described earlier or the like, a GaN
substrate including low-defect regions is obtained. In this
embodiment, the substrate is assumed to have a defect density of
about 10.sup.6 cm.sup.-2 or less over the entire area thereof.
Next, all over the surface of this substrate, SiO.sub.2 or the like
is vapor-deposited so as to have a thickness of 1 .mu.m by
sputtering. Then, by common photolithography; stripe-shaped windows
are formed with photoresist so as to have a width of 40 .mu.m each
and a period of 400 .mu.m in the [1-100] direction. Then, by ICP or
RIE (reactive ion etching), the SiO.sub.2 and the GaN substrate are
etched. The GaN substrate is etched to a depth of 6 .mu.m.
Thereafter, the SiO.sub.2 is removed with an etchant such as HF.
This is the end of the treatment of the substrate to be performed
before a nitride semiconductor film is grown thereon.
[0048] FIGS. 2A and 2B show the thus obtained substrate, in a top
view and a sectional view, respectively. Reference numeral 21
represents the GaN substrate, and reference numeral 22 represents
the regions etched by RIE. Symbols X, Z, and T represent the
etching width, depth, and period, respectively. The etching may be
achieved by the use of gas-phase etching, or by the use of a liquid
etchant. In the following descriptions, the regions 22 of the
substrate depressed as a result of being removed by etching will be
referred to also as the carved regions. The carved regions may be
formed after once the thin film of GaN, InGaN, AlGaN, InAlGaN, and
the like has been grown on the GaN substrate including low-defect
regions. That is, the present invention includes structures wherein
a nitride semiconductor film is grown by first growing it and then
forming carved regions.
[0049] The carved regions may be arranged in one of various
patterns. For example, as shown in FIGS. 8A and 8B, two carved
regions may be formed at a predetermined interval; or, as shown in
FIGS. 9A to 9C, more than two carved regions may be formed, or
carved regions may be formed with different periods used mixedly,
or carved regions may be formed in a mixed pattern including singly
and doubly arranged carved regions. The present invention is
applicable as it is so long as the cross-sectional area and period
of one carved region is within the ranges defined in the appended
claims. In cases where different periods are used mixedly, each of
those mixedly used periods needs to be within the range defined in
the appended claims. Here, reference numerals 81 and 91 represent
the GaN substrate, and reference numerals 82 and 92 represent the
carved regions.
[0050] In FIGS. 1A, 1B, 2A, 2B, 5, 7A, 7B, 8A, and 8B, the carved
regions are formed parallel to the [1-100] direction. It is,
however, also possible to form them in another direction, for
example parallel to the [11-20] direction. Basically, the effects
of the present invention do not depend on the direction of carving,
and therefore the carved regions may be formed in any
direction.
[0051] The substrate used may include a region with high defect
density. This, however, may lead to degraded surface morphology
during epitaxial growth, and therefore it is preferable to use a
substrate including no region with high defect density.
[0052] On this substrate, the nitride semiconductor film shown in
FIG. 4 is grown, and thereby the nitride semiconductor laser device
of this embodiment is obtained. FIGS. 1A and 1B schematically show
the structure of the thus obtained semiconductor laser device. FIG.
1A is a sectional view of the semiconductor laser device as seen
from the direction in which it emits light, and FIG. 1B is a top
view of the semiconductor laser device as seen from above.
[0053] Here, reference numeral 10 represents the GaN substrate,
and, in this substrate 10, low-defect regions are present. FIG. 4
shows the following. On an n-type GaN layer (1.0 .mu.m) 40, there
are laid the following layers one on top of another in the order
mentioned: an n-type Al.sub.0.062Ga.sub.0.938N first clad layer
(1.5 .mu.m) 41, an n-type Al.sub.0.1Ga.sub.0.9N second clad layer
(0.2 .mu.m) 42, an n-type Al.sub.0.062Ga.sub.0.938N third clad
layer (0.1 .mu.m) 43, an n-type GaN guide layer (0.1 .mu.m) 44, an
InGaN/GaN-3MQW active layer (InGaN/GaN=4 nm/8 nm) 45, a p-type
Al.sub.0.3Ga.sub.0.7N evaporation prevention layer (20 nm) 46, a
p-type GaN guide layer (0.05 .mu.m) 47, a p-type
Al.sub.0.062Ga.sub.0.938N clad layer (0.5 .mu.m) 48, and a p-type
GaN contact layer (0.1 .mu.m) 49.
[0054] On top of the substrate 10, a nitride semiconductor film
(epitaxially grown layer) 11 is formed that has the same structure
as the nitride semiconductor thin film shown in FIG. 4. Moreover,
on the top surface of the nitride semiconductor film 11, a laser
stripe 12 is formed as a laser light waveguide structure. This
laser stripe 12 needs to be formed so as to be located above a
low-defect region included in the substrate. The substrate used in
this embodiment has low-defect regions all over the area thereof,
and therefore the laser stripe may be formed anywhere thereon
except above carved regions. The reason will be described
later.
[0055] On the top surface of the nitride semiconductor film 11 is
formed SiO.sub.2 13 for current narrowing, and on the top surface
of this is formed a p-type electrode 14. On the other hand, on the
bottom surface of the substrate 10 is formed an n-type electrode
15. Reference numeral 16 represents a carved region. The top
surface of the portion of the nitride semiconductor film 11 located
above the carved region 16 is depressed under the influence of the
carved region 16.
[0056] Whether the top surface of the portion located above the
carved region 16 is depressed or not depends on the thickness of
the nitride semiconductor film. FIG. 7 schematically shows the
structure obtained when the top surface of the portion located
above the carved region is flat. In FIG. 7, reference numeral 70
represents the GaN substrate, reference numeral 71 represents the
nitride semiconductor film, reference numeral 72 represents the
laser stripe, reference numeral 73 represents SiO.sub.2 for current
narrowing, reference numeral 74 represents the p-type electrode,
reference numeral 75 represents the n-type electrode, and reference
numeral 76 represents the carved region. In this way, as the
nitride semiconductor film is made thicker, the top surface of the
portion located above the carved region 76 becomes flatter. It
should be noted that, in the present invention, whether the top
surface of the portion located above the carved region is depressed
or flat does not matter.
[0057] In FIG. 1A, the distance from the center of the laser stripe
12 to the edge of the carved region 16 is represented by "d", and
specifically d=40 .mu.m here. FIG. 5 schematically shows a top view
of the wafer before it is diced into individual semiconductor laser
devices. In this embodiment, it was possible to obtain a nitride
semiconductor film 51 completely free from cracks all over the area
thereof. In FIG. 5, reference numeral 52 represents the carved
regions.
[0058] The wafer can be diced into individual nitride semiconductor
laser devices by a common dicing process. No description will be
given of this dicing process. No cracks were observed in the
nitride semiconductor laser devices after the separation of the
wafer into individual chips. As a result, the laser devices
oscillated with stable characteristics, and the yield rate of the
semiconductor laser devices of this embodiment which offered the
desired oscillation characteristics (i.e., those requiring a drive
current lop of 70 mA or less when producing an optical output of 30
mW) was more than 90%.
[0059] The useful lives of the diced semiconductor laser devices
were tested with the devices driven under APC (automatic power
control) at 60.degree. C. and at an output of 30 mW. In the test,
the devices emitted at wavelengths of 405.+-.5 nm. From each wafer,
50 semiconductor laser devices that fulfilled predetermined initial
characteristics were randomly picked out, and the number of devices
of which the useful lives exceeded 3,000 hours was counted as the
yield rate. Here, the yield rate of the semiconductor laser device
of this embodiment was more than 85%.
Comparative Example 1 of a Semiconductor Laser Device
[0060] Now, with reference to FIG. 3, a description will be given
of what happened when, without any extra treatment performed, a
nitride semiconductor film was grown on a substrate including
low-defect regions in the surface thereof. Reference numeral 31
represents a wafer produced by growing, by MOCVD, a nitride
semiconductor thin film as shown in FIG. 4 on a substrate including
low-defect-density regions. Reference numeral 32 represents cracks
that developed in the wafer.
[0061] When a nitride semiconductor film was grown on a substrate
including low-defect regions as it is (i.e., if no carved regions
were formed thereon), as shown in FIG. 3, many cracks developed in
the wafer. The result of counting the number of cracks that crossed
a 1 mm.times.1 mm region on the wafer was about three to ten. If a
fabricated device contains cracks, the device may be flatly unable
to produce laser oscillation at all, or, even if it can, its useful
life is extremely short, making the device practically unusable.
For this reason, the yield rate of devices that produced the
desired laser oscillation was extremely low, specifically 50% or
less. The development of such cracks is remarkable in a device
structure including a layer containing Al, and, since a nitride
semiconductor laser device typically includes such a layer, it is
very important to eradicate cracks.
[0062] Moreover, when semiconductor laser devices were fabricated
in portions of the wafer where no cracks happened to develop and
their useful lives were tested at 60.degree. C. and at 30 mW, the
yield rate of devices with a useful life of 3,000 hours or more was
as poor as about 15%. One cause for this is considered to be minute
cracks that are present in the wafer but that cannot be observed
from the surface of the wafer. Here, the useful life is defined as
the length of time required for the drive current Iop to become 1.5
times the initial level thereof while the output is kept at 30
mW.
[0063] The embodiment under discussion aims to obtain a long useful
life by reducing cracks, increasing the yield rate of semiconductor
laser devices, and controlling the strains that develop therein.
Now, the embodiment will be described in detail.
Comparative Example 2 of a Semiconductor Laser Device
[0064] Another comparative example of a semiconductor laser device
was fabricated in which a laser stripe was formed above a carved
region. Except for the position of the laser stripe, this
semiconductor laser device has the same structure as that of the
embodiment under discussion. The useful lives of semiconductor
laser devices diced out of a wafer having a structure wherein laser
stripes were formed right above carved regions 16. In this test,
the yield rate was 35% or less. The lessening of the yield rate
here is believed to result from severer strains contained in the
semiconductor laser devices. The fact that, when no carved regions
were formed in the substrate, many cracks developed is considered
to suggest the presence of considerably severe strains.
[0065] In the portion above a carved region, the film grows
horizontally from the noncarved portions contiguous therewith, and
thus the film runs into the carved portion. At this time, pressure
acts from both sides on the portion above the carved region, and
this is considered to cause this portion to contain severer strains
than noncarved regions. Moreover, the carved region has walls at
both sides, and therefore the growth that tends to advance to both
sides is hampered by the walls. This also results in contained
strains. Growth in the carved region is complicated; specifically,
growth advances from different directions (normal growth that
advances from the bottom surface of the carved region, growth that
advances from the side faces of the carved region, growth resulting
from running-in from noncarved regions, etc.). Thus, not only does
the severity of strains vary within a region, the direction of
strains also differs from one place to another, resulting in poor
repeatability and thus instability. This is considered to be the
cause of the lessening of the yield rate.
[0066] Moreover, since growth advances from different directions,
many dislocations, defects, and the like develop where growth from
different directions meets. Accordingly, when laser stripes are
formed above carved regions, such dislocations, defects, and the
like promote deterioration, making it impossible to obtain long
useful lives.
[0067] On the other hand, a noncarved region, when growing, runs
into carved regions, and can thereby release strains. This
releasing of strains suppresses development of cracks, and
simultaneously releases the strains contained in the noncarved
region. This releasing of strains occurs with good repeatability
and with stability. Moreover, as opposed to the portion above a
carved region, growth does not advance from different directions.
This helps produce a satisfactorily crystalline film free from
dislocations, defects, and the like. These are believed to be the
reasons that forming laser stripes in noncarved regions increases
the reliability of semiconductor laser devices and prolong the
useful lives thereof.
[0068] In this embodiment, by forming carved regions 16 and forming
laser stripes 12 elsewhere than right above the carved regions 16,
it is possible to greatly increase the reliability of LD devices,
to suppress the development of cracks, and to dramatically improve
the yield rate.
Study on the Carving Conditions and the Layer Thickness
[0069] Moreover, the inventors of the present invention have found
out that the yield rate correlates with the carving width X (see
FIG. 2B) of the carved regions, the carving depth Z thereof, and
the total film thickness of the nitride semiconductor film grown on
the substrate. The total film thickness of the nitride
semiconductor film is the total thickness of all the layers thereof
including from the n-type GaN layer 40 through the p-type GaN
contact layer 49 shown in FIG. 4.
[0070] Here, the total film thickness was adjusted so as to be
varied from 2 .mu.m to 30 .mu.m. FIG. 6 shows the results of
plotting the yield rate measured by the useful life test described
above against different combinations of the carving cross-sectional
area, i.e., the carving depth Z (in .mu.m) multiplied by the
carving width X (in .mu.m), and the total film thickness of the
nitride semiconductor film grown on top thereof. The period T (see
FIG. 2B) of the carved regions was 400 .mu.m.
[0071] The regions in which the yield rate was found to be high by
the useful life test indicate that, there, the development of
cracks was effectively suppressed and the strains in noncarved
regions (where ridges are formed) were effectively released. With
the carving width X=10 .mu.m and the carving depth Z=5 .mu.m, the
carving cross-sectional area equals 5.times.10=50 .mu.m.sup.2. The
carving width X was varied in the range from the 3 .mu.m to 200
.mu.m, and the carving depth Z was varied in the range from 0.5
.mu.m to 30 .mu.m.
[0072] As shown in FIG. 6, when the carving cross-sectional area
was 30 .mu.m.sup.2 or more, it was possible to obtain high yield
rates irrespective of the total film thickness of the nitride
semiconductor film grown on the substrate. This is considered to be
because the strains in noncarved regions were effectively and
stably released. Although FIG. 6 only covers up to a
cross-sectional area of 100 .mu.m.sup.2, it was in fact possible to
obtain yield rates of 80% or more up to a cross-sectional area of
2,000 .mu.m.sup.2 in the above film thickness range.
[0073] When the carving cross-sectional area was in the range from
5 .mu.m.sup.2 to 30 .mu.m.sup.2, so long as the total film
thickness of the nitride semiconductor film grown on the substrate
was 10 .mu.m or less, it was possible to obtain improved yield
rates. When the carving cross-sectional area was less than 5
.mu.m.sup.2, no improvement was obtained if the total film
thickness of the nitride semiconductor film was in the range from 2
.mu.m to 30 .mu.m. This is considered to be because the carving
cross-sectional area was so small that the strains in noncarved
regions were not released effectively.
[0074] FIG. 6 shows the results obtained when the period T of the
carved regions was 400 .mu.m. Similar tests were conducted with
varying periods T. The period T was varied in the range from 50
.mu.m to 2 mm. The carving width X was varied in the range up to
one-half of the period T. For example, when the period T was 50
.mu.m, the carving width X was varied in the range from 0 to 25
.mu.m.
[0075] When the period T was in the range from 50 .mu.M to 2 mm, a
tendency approximately identical with that shown in FIG. 6 was
observed. Specifically, as shown in FIG. 6, when the carving
cross-sectional area was 30 .mu.m.sup.2 or more, it was possible to
obtain high yield rates irrespective of the total film thickness of
the nitride semiconductor film grown on the substrate; when the
carving cross-sectional area was in the range from 5 .mu.m.sup.2 to
30 .mu.m.sup.2, so long as the total film thickness of the nitride
semiconductor film grown on the substrate was 10 .mu.m or less, it
was possible to obtain improved yield rates; when the carving
cross-sectional area was less than 5 .mu.m.sup.2, no improvement
was obtained if the total film thickness of the nitride
semiconductor film was in the range from 2 .mu.m to 30 .mu.m.
Study on the Position of the Stripe
[0076] With respect to where to form ridges, forming them at a
distance of 5 .mu.m or less from the edges of carved regions 16
resulted in greatly shortened useful lives in the useful life test.
This is considered to be because severe strains were present around
carved regions. Accordingly, laser stripes need to be formed 5
.mu.m or more away from the edges of carved regions. Moreover, the
position of laser stripes needs to be so determined that they are
formed not only in regions with mild strains but in regions with
high flatness.
[0077] A substrate including carved regions suffers from variations
in the thickness of the epitaxially grown layer which are observed
around the carved regions. FIG. 10 is a diagram illustrating this
situation, and shows the state of a wafer 1001 produced by
epitaxially growing, by MOCVD, a plurality of nitride semiconductor
layers (for example, with a total film thickness of 5 .mu.m) on a
GaN substrate having carved regions 1002 formed thereon so as to be
parallel to one another. In the regions between the grooves, there
are formed semiconductor laser waveguide stripes 1004 (of which the
position is indicated by broken lines).
[0078] It is inevitable that, under the influence of the grooves,
the film thickness of the grown layer varies according to the
distance from the grooves. In reality, however, evaluating the
layer thickness even at a fixed distance from a groove along the
direction of the grooves reveals variations in the layer thickness.
Moreover, when the surface of the wafer is inspected under an
optical microscope, wave-like morphology 1005 is observed as
schematically shown in the figure.
[0079] This is considered to be because how the crystal grows in
the regions 1003 between the carved regions is sensitively
influenced by the grooves. If laser waveguide stripes are formed in
such regions, variations in the film thickness along the wave
guides not only adversely affect the laser characteristics but also
make the characteristics of individual devices uneven.
[0080] By contrast, in regions 30 .mu.m or more away from the edges
of the carved regions, the above-described variations in the film
thickness of the grown layer rapidly diminish, making wave-like
surface morphology as shown in FIG. 10 unobservable.
[0081] Along the direction of arrow X shown in FIG. 10, height
variations on the surface were measured by using a height
difference measurement machine. The measurements were made by using
the "DEKTAK3ST" model manufactured by A SUBSIDIARY OF VEECO
INSTRUMENTS INC. The measurement conditions were: measurement
length, 2,000 .mu.m; measurement duration, 3 minutes; probe needle
pressure, 30 mg; horizontal resolution, 1 .mu.m per sample. FIG. 11
shows the results of measuring height variations in noncarved
regions 30 .mu.m away from carved regions. FIG. 12 shows the
results of measuring height variations in noncarved regions 5 .mu.m
away from carved regions. As will be understood from FIGS. 11 and
12, whereas height variations on the surface in regions 30 .mu.m
away from carved regions were about 40 nm, those in regions 5 .mu.m
away therefrom were as large as 200 nm.
[0082] In a semiconductor laser, the laser stripe needs to be
formed at a certain distance, 5 .mu.m or more at least (preferably,
30 .mu.m or more), to suppress variations (strains and flatness)
resulting from the influence of grooves as described in connection
with the related art. In such a position, lateral growth does not
effectively suppress the propagation of defects from the
substrate.
[0083] The purpose of forming grooves in the substrate according to
the present invention is utterly different from the purpose of
forming grooves in a substrate with a view to exploiting so-called
lateral growth technology (for example, ELOG technology) to reduce
the density of defects extending from the substrate to a crystal
growth film. For the purpose of reducing the defect density, to
obtain the effect of lateral growth, the intervals between the
grooves are typically about equal to the film thickness of the
formed layer or less, and are, even extended to the maximum, about
three times the film thickness or less. In this structure, it is
difficult to obtain regions where, as described above, the layer
thickness is uniform in the direction parallel to the grooves.
Thus, when laser stripes are formed, undesirably, the film
thickness varies in the direction of the stripes.
[0084] By contrast, the grooves in the present invention are formed
not for that purpose, but for the purpose of maintaining a certain
degree of flatness where the laser stripe is formed while
effectively preventing cracks. The intervals of the grooves are
about of the same order as the width of the semiconductor laser
device, specifically about 50 .mu.m at the minimum, and preferably
100 .mu.m or more.
[0085] Here, the description deals specifically with a
semiconductor laser. It should be understood, however, that
application of the present invention is not limited to this
particular type of device. For example, even in a case where an
electronic device such as a light-emitting diode (LED) or FET
(field-emission transistor) is formed on a substrate as described
in connection with this embodiment, on the same principles as those
described above, it is possible to greatly reduce strains and
cracks present in a nitride semiconductor film and thereby increase
the yield rate. With an LED or the like, problems have been
reported such as an uneven light emission pattern and lowering of
light emission intensity caused by strains present in a film.
[0086] In such a device, carved regions 131 may be formed in the
shape of stripes that run both longitudinally and laterally so as
to form a mesh-like pattern as shown in FIGS. 13A and 13B. In FIGS.
13A and 13B, reference numeral 132 represents an n-type GaN
substrate, reference numeral 133 represents a p-type electrode,
reference numeral 134 represents an n-type electrode, reference
numeral 135 represents a nitride semiconductor thin film. When an
LED was fabricated with the structure shown in FIGS. 13A and 13B,
it was possible to reduce the strains present in the nitride
semiconductor film, to alleviate the unevenness in the light
emission pattern, and reduce cracks to zero.
[0087] Obviously, many modifications and variations of the present
invention are possible in light of the above teachings. It is
therefore to be understood that within the scope of the appended
claims, the invention may be practiced other than as specifically
described.
* * * * *